1. Field of the Invention
This invention relates generally to power supplies and, more specifically, to DC supplies that provide constant power to a resistive load over a broad range of resistance values.
2. Description of the Prior Art
In telephone system applications, particularly those relating to analog or digital carrier systems operating over subscriber loops, a major design consideration is one of limiting power dissipation. Generally, a subscriber loop carrier system must supply DC current to the telephone equipment which it serves. Dissipation becomes a significant concern in a system wherein many loops are powered within a small volume, such as a field-located cabinet. In addition, dissipation increases reserve battery capacity requirements and cost. Therefore a nondissipative DC power supply that provides constant average power into a variable resistance load allows for effective power load planning and equipment design.
Many prior art supplies employ dissipative and, oftentimes, complex circuitry, including active components, to achieve the desired regulated output power over the anticipated range of load resistances. The cost of such an arrangement is an important factor in the overall cost of the system and thus affects the prove-in point of the system. Moreover, because of the remote location and harsh operating environment of the cabinet, maintenance and reliability also present problems. A supply developed from circuitry utilizing passive elements offers reduced cost and maintenance and improved reliability.
The article entitled "Constant Voltage, Current, and Power: A Circuit for All Reasons," by D. A. Paice, IEEE Transactions on Industrial Electronics and Control Instrumentation, February 1978, is representative of prior art power supplies that employ an alternating polarity source and passive elements, including rectifying means, to derive a DC supply. The article discloses circuit arrangements and corresponding component values for obtaining a relatively constant power output over a range of load resistance values. However, the range is somewhat limited since, for example, only a three octave change in resistance values about a preselected operating point results in .+-.35 percent power variations. Furthermore, the "constant" power mode is established by tuning an inductor and capacitor pair to the frequency of a sinusoidal input source so operation is sensitive to component variations of this pair.